1. Field of the Invention
The present invention relates to a detecting technique for identifying a defective recording element from a test pattern recording result produced by a recording head having a plurality of recording elements (for example, an inkjet head), and an image forming technique to which the detecting technique is applied.
2. Description of the Related Art
Methods of recording an image on a recording medium such as a recording paper include an inkjet rendering method in which ink drops are ejected from a recording head in accordance with an image signal in such a manner that the ink drops are landed on the recording medium. An example of an image rendering apparatus using such an inkjet rendering system is a full-line head image rendering apparatus in which an ejecting unit (a plurality of nozzles) that ejects ink drops is linearly disposed to correspond to an entire area of one side of a recording medium, and the recording medium is conveyed in a direction perpendicular to the ejecting unit in order to enable an image to be recorded on an entire area of the recording medium. Since a full-line head image rendering apparatus is capable of rendering an image on an entire area of a recording medium by conveying the recording medium without moving an ejecting unit, the full-line head image rendering apparatus is suitable for increasing recording speed.
However, with a full-line head image rendering apparatus, a deviation of an actual dot position that is recorded on a recording medium from an ideal dot position due to various reasons such as production variation, deterioration with age, or the like of recording elements (nozzles) constituting an ejecting unit may cause a recording position error (landing position error). As a result, a problem arises in that streaky artifacts occur in an image recorded on the recording medium. In addition to artifacts due to such a recording position error, there are phenomena in which streaky artifacts occur in a recorded image on the recording medium due to failures in a recording element such as an abnormality in which droplets are not ejected (non-ejection), an abnormality in ejection volume, and an abnormality in ejection shape (splash). Such recording elements which cause a decline in recording quality are collectively referred to as “defective ejection nozzles” or “defective recording elements”. Since a length of a full-line recording head is equivalent to a width of a recording paper, for example, when recording resolution is 1200 DPI, recording elements of an apparatus capable of accommodating a recording paper having a paper width similar to that of half Kiku size (636 mm by 469 mm) number approximately 30,000 nozzles per ink. With such a large number of recording elements, defective ejection nozzles may occur at various timings. More specifically, a nozzle may become defective at the time of manufacture of a recording head, a nozzle may become defective due to deterioration with age, a nozzle may become defective during maintenance (when maintenance-induced, the nozzle is often restored to a normal nozzle by a next maintenance), and a recording element may become a defective ejection nozzle midway through continuous printing.
A technique is known in which, when a defective ejection nozzle occurs, usage of the defective ejection nozzle is suspended (ejection suspension) and other surrounding nozzles (nozzles capable of normal ejection) are used in order to correct an image. When applying such a correction technique, it is required that a defective ejection nozzle is accurately identified.
As techniques for identifying a defective ejection nozzle, Japanese Patent Application Publication No. 2004-009474, Japanese Patent Application Publication No. 2006-069027, and Japanese Patent Application Publication No. 2007-054970 describe methods of identifying a defective ejection nozzle by printing a predetermined test pattern aimed at detection of a defective ejection nozzle, reading a printing result with an image reading apparatus, and analyzing obtained read image data.
Japanese Patent Application Publication No. 2004-009474 discloses a configuration that uses a 1-on N-off detection test pattern. A reading apparatus (scanner) has a resolution that is equal to or higher than a print resolution, and binarizes a read result and detects a non-ejection nozzle.
In addition, Japanese Patent Application Publication No. 2006-069027 discloses a technique for detecting a defective nozzle position based on an average value of read results of a single row of interest among a test pattern and an average value of read results of m-number of rows that are to the left and right of the row of interest. In this case, it is assumed that a reading resolution of an image reading unit is favorably n-times a resolution of a line head (where n is a natural number equal to or greater than 2).
As described above, both Japanese Patent Application Publication No. 2004-009474 and Japanese Patent Application Publication No. 2006-069027 do not disclose a detection technique that addresses a problem of using a reading apparatus having a lower resolution than a line head.
In light of this problem, Japanese Patent Application Publication No. 2007-054970 discloses a technique involving using a scanner that reads at a lower resolution than a resolution of a recording head and interpolating read data to detect a defective nozzle.
However, the technique described in Japanese Patent Application Publication No. 2007-054970 has a problem in that since a certain amount of error (an estimation error of a line profile formed by dots) remains on a line position under a condition in which a width of a line formed by dots on a test pattern does not satisfy a sampling theorem, accuracy is not sufficiently high.